Conventionally, a ship carries seawater, which is drawn up by a pump or the like, as ballast water in a tank provided in the bottom of the ship and, when arrived at its destination, discharges the ballast water to the outside while loading and unloading cargos. This is because, when the ship does not carry any cargos or when a quantity of cargos on the ship is a small, a draft line is lowered which makes it unable to keep its balance. An overtime in pumping-up and discharging operations leads to an increase in transportation cost. Therefore, in order to finish the operation within the loading and unloading time of the cargos, the pumping-up and discharging operations are generally carried out at high speeds such as several hundreds of tons per hour to several thousands of tons per hour.
Usually, a marine area in which the ballast water is taken in and a marine area in which the ballast water is discharged are different. Depending on the marine area in which the seawater is taken in, harmful plankton or bacteria may be mixed therein, and if it is discharged in the coastal area or inside a harbor at the destination, it would mean that the harmful plankton or the bacteria are artificially brought in from the intake marine area to the discharging marine area. In some circumstances, they may proliferate abnormally in the discharged marine area and destroy the ecosystem, and may cause shellfish poisoning or a red tide to cause marine pollution, which would inflict a serious blow to the fishing industry. In fact, such cases are actually being reported. Further, they can harm even public health.
In order to prevent such harmful plankton or bacteria from being transported in accordance with the movement of the ship, in February 2004, the International Maritime Organization (IMO) adopted an International Convention for the Control and Management of Ships' Ballast Water and Sediments. In this convention, a strict discharge standard on the ballast water is provided, which is referred to as the D2 ballast water discharge standard, according to which dischargeable amounts are regulated as less than 10 individuals/ton with respect to the number of plankton having a size of more than 50 μm in the ballast water of 1 ton, less than 10 individuals/1 ml with respect to the number of plankton of 10 μm to 50 μm, and less than 250 cfu (colony forming unit)/100 ml with respect to Escherichia coli. Therefore, according to this discharge standard, microorganisms ranging from large-sized plankton of more than 50 μm to small bacteria of about 1μ are required to be removed simultaneously and almost entirely; and also, vast amount of ballast water is required be treated in a short time.
At present, in countries such as Australia and the U.S., this strict discharge standard is being implemented in advance. Thus, most ships heading for these countries carry out ballast water exchange in a coastal area such as an area near a harbor in order to meet this strict discharge standard. That is, before calling at the harbor, the ballast water that has been carried onboard the ship is exchanged with clean and non-problematic oceanic water (reballast method).
However, when exchanging the ballast water, balance keeping of a hull of the ship becomes a problem, which may induce a capsizing accident due to a procedural error in the exchanging operation. Thus, ensuring safety upon the exchanging operation is regarded as an issue. Besides, the exchange of the ballast water does not essentially solve the problem. Moreover, the standard itself is moving in a stricter direction, like in countries such as Australia and the U.S., it is being considered to set stricter criteria.
So far reported ballast water treatment apparatuses under development are centered on chemical treatment methods in which a medical treatment with a chemical agent, such as chlorine, or ozone is used to carry out disinfection or to eradicate plankton and bacteria, following the G9 standard which is based on a chemical approach. That is, under the current situation, reliance on the chemical treatment has been inevitable in order to clear the D2 ballast water discharge standard. Here, in the present description, the term “sterilization” means removing microorganisms such as plankton and bacteria to a target level.
As described above, the taking in and discharging operations of the ballast water are carried out while loading and unloading operations. Thus, if the ballast water is disinfected in accordance with the G9 standard, the ballast water treated with a chemical agent would be discharged in a large quantity into the harbor, in which case the chemical agent may give an extensive influence to the living creatures in the harbor and destroy the ecosystem. Therefore, there are built up expectations on a method which complies with the G8 standard relating to apparatuses and which carries out sterilization or disinfection using a physical treatment.
Filtration is a typical physical treatment. As means for filtration, a strain filtering phenomenon in which a filter medium, such as a strainer, catches solid particles that are larger than the voids (openings) formed therein, a depth filtering phenomenon in which voids formed in sterically layered cakes, sands and the like catch solid particles, and a cake filtering phenomenon in which surfaces of cakes, which grow in the course of filtration, separate large solid particles and allow only clean water through the cakes for the filtration, are mainly utilized.
In order to obtain clean water by filtering out even small bacteria of about 1μ, the eyes of the filter medium need to be small. Hence, a clogging occurs in a short time. For filtration in this case, it can be considered to utilize the cake filtering phenomenon described above, however, since the pressure loss (fluid resistance) becomes high immediately, it difficult to sterilize the ballast water flowing at a flow rate of several hundreds of tons/hour or several thousands of tons/hour. Accordingly, the chemical agent treatment described above, a heat treatment or an electrical treatment has been indispensable.
For example, when the ballast water is thermally treated at a temperature of about 60° or higher, there can be produced an environment which is unsuitable for Escherichia coli and the like to survive (heat treatment method). As specific methods, it can be considered to introduce heated engine cooling water into a tank, or to install a heat exchanger in a ballast water piping system. However, it cannot be said that these methods can reliably carry out disinfection and, besides, energy efficiency is not good. That is, large amount of heat and cost are required for disinfection. Further, carrying the hot water onboard affects the cargos, and an influence of the discharge of the hot water to marine creatures is also a concern.
Other methods than the heat disinfection are also being proposed, such as disinfection using ozone or mixture of ozone and steam (ozone method), disinfection by electrically destroying microorganisms using a high voltage pulse or the like, disinfection using ultraviolet rays or a photocatalyst (ultraviolet method), an electrochemical method, etc. However, neither of the proposals are practically satisfactory.
Also, as a composite method combining the above methods, there is proposed a method in which a filtration with a filtration medium pre-coated with a filter aid is combined with a disinfection using a heat treatment or an electric treatment (see, e.g., Patent Document 1). In this method, the filter aid forms a thin film on a surface of the filter medium, and since the filtered out microorganisms gather on this layer, heat energy or electric energy is applied to the gathered region to effectively carry out the disinfection. However, this is still difficult to treat a large amount of ballast water, and is difficult to say that it is practical also in view of cost.
There are types of filter medium, some aiming at microfiltration capable of filtering out suspended solids of 100 μm or less, and some aiming at general filtration for filtering out larger sizes. A sintered filter is used for microfiltration, whereas a spring filter is used for general filtration. The spring filter makes it possible to relatively lower a pressure loss. Further, when the filter medium is pre-coated with the filter aid, filtration utilizing a cake filtering phenomenon becomes possible. As a filtration apparatus of this type, there is known a filter in which pre-coat aids are layered on the filter medium sequentially in such order as they increase in the particle diameter thereof (see Patent Document 3).
A coil of the spring filter is configured such that protrusions are provided at given intervals on a spiral side face of the coil so that the gaps of the coil have a constant value (see, e.g., Patent Document 2). However, in order to remove small bacteria, these gaps need to be considerably narrow, which inevitably increases fluid resistance. When the gaps of the coil are widened, the filter aid becomes smaller and, thus, flows away through the gaps.
Patent Document 1: JP 2006-102283 A
Patent Document 2: JP 52-90871 A
Patent Document 3: JP 3-38117 U